The present invention relates to an electromagnetic actuator having at least one magnet and coil pair for generating a desired actuation force in a device. More particularly, the invention relates to such an actuator which comprises a core having central pole which is specifically designed to reduce the inductance of the coil and thereby permit the actuator to generate a greater force while remaining within the inductance and current limits pertaining to intrinsically safe devices.
In the field of industrial process control, devices such as flowmeters, densitometers, valve positioners and the like often include an electromagnetic actuator for generating a desired force which is required to operate the device. The electromagnetic actuator typically comprises at least one coil which is paired with a magnet that is mounted in a core which serves to direct the flux lines of the magnetic field produced by the magnet across the coil. Thus, when the coil is energized, the current in the coil will interact with the magnetic field to generate a force that acts between the coil and the magnet. For example, vibrating tube-type mass flowmeters and densitometers typically employ one or more magnet and coil pairs to generate vibratory excitation forces on the tube through which the material to be measured or processed is passed.
The force which is generated by the magnet and coil pair is proportional to both the current in the coil and the flux density of the magnetic field existing in the air gap in which the coil is located. Furthermore, although the flux density can be increased, prior art electromagnetic actuators are typically designed to have a flux density in the core of no more than about 50% of the saturation flux density of the core material. This is due to the fact that, heretofore, increasing the flux density usually required using a larger magnet or one which is constructed of a more expensive material, and this would unduly increase the weight or cost of the actuator. Thus, devices which require large actuating forces usually must be powered by high operating currents.
Since many industrial applications require that these devices operate in hazardous environments in which flammable or explosive gases or dust may be present, certain approval agencies such as UL, CSA, BVS and the like have developed safety regulations governing the design of these devices in order to minimize the risk of explosion. One method by which a device can be qualified to operate in hazardous environments is to design the electromagnetic actuator to meet the regulations applicable to xe2x80x9cintrinsically safexe2x80x9d (xe2x80x9cISxe2x80x9d) devices. The IS regulations require that the amount of stored energy in the electromagnetic actuator, which is proportional to both the inductance of the coil and the square of the current in the coil, be minimized by limiting the current and inductance levels so that a failed component or broken or shorted wire will not cause a spark which could trigger an explosion. The various approval agencies publish design guidelines delimiting the maximum acceptable current levels for given inductance values for IS devices. However, certain industrial devices which require large electromagnetic actuating forces necessitate the use of currents which are normally greater than the maximum acceptable current levels. Consequently, such devices normally cannot be qualified for use in hazardous environments as IS devices.
According to the present invention, these and other disadvantages in the prior art are overcome by providing an electromagnetic actuator for an IS device having a first component which is movable relative to a second component. The electromagnetic actuator comprises a coil which is coupled to the first component, a magnetically permeable core which is coupled to the second component, a magnet which is positioned in association with the coil and which is magnetically coupled to the core, and a power supply for selectively energizing the coil. The core preferably includes a cylindrical keeper which comprises first and second ends, an annular pole which is positioned concentrically within the keeper and which comprises first and second ends, and a base which connects the first end of the keeper to the first end of the pole. The inner diameter of the keeper is greater than the outer diameter of the pole, and the coil is positioned between the keeper and the pole. In one embodiment of the invention, the magnet comprises a cylindrical body having an outer diameter which is affixed to the keeper and an inner diameter which is greater than the outer diameter of the pole to thereby define a gap between the pole and the magnet in which the coil is positioned. Moreover, in accordance with the present invention the shape of the pole is designed to increase the flux density in the pole and thereby reduce the permeability of the pole, which consequently will reduce the inductance of the coil.
According to a preferred embodiment of the invention, the pole comprises an inner surface which is tapered from the first end to the second end to achieve a flux density in the pole which is greater than 50%, and preferably between about 75% to 95%, of the saturation flux density of the pole. It has been found that the permeability of the pole at this level of flux density is sufficiently reduced to allow a meaningful decrease in the inductance of the coil while still permitting the electromagnetic actuator to operate at an acceptable efficiency level. Thus, a higher current can be employed in the coil while still maintaining the total stored energy in the electromagnetic actuator within required limitations. Therefore, the electromagnetic actuator is capable of generating a greater actuating force than was previously possible while allowing the device to operate within the current and inductance limitations prescribed by the IS regulations.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.